Abrasive and grinding wheel



ABRASIVE AND GRINDING WHEEL Loring Coes, Jr., Brookfield, Mass., assignor to Norton Company, Worcester, Mass., :1 corporationof Massachusetts No Drawing. Application March 10, 1953, Serial No. 341,595

4 Claims. Cl. 51 -298) The invention relates to abrasives and grinding wheels.

One object of the invention is to provide a superior grinding wheel for snagging operations. Another object of the invention is to provide a grinding wheel suitable for use in a power controlled billet grinder. In this connection billet grinding has heretofore been done principally with so-called swing frame grinders in which the operator had to move and control the wheel by his own physical strength. Weights were used to give the grinding pressure but the resultant forces had to be counteracted by the operators strength. Recently power operated billet grinders have been developed in which the operator merely moves valves or the like to move the grinding wheel to different parts of the billet and, since the force to move the grinding wheel is hydraulic, great pressures can be used. Such machines are described in copending applications of my colleague George Comstock, 3rd, Serial No. 285,423, filed May 1, 1952, and Serial No. 287,530, filed May 13, 1952, also in copending applications of my colleague Carl Soderlund, Serial No. 304,170, filed August 13, 1952, and Serial No. 312,715, filed October 2, 1952.

Another object of the invention is to provide a grinding wheel capable of being used at very high speeds, that is to say having a breaking strength as measured in surface feet per minute materially higher than billet grinding wheels or other wheels of this general type heretofore made. Another object of the invention is to provide a grinding wheel, for use for example as a billet grinding wheel, which can be usedat high pressures and under such conditions to give superior grinding results as measured in the conventional manner.

Other objects will be in part obvious or in part pointed out hereinafter.

I have found that an abrasive, superior to fused alumina for some purposes, can be made by the following process:

Calcined bauxite is ball milled to 20 hours in a steel ball mill. The fine powder is then stirred into 10% hydrochloric acid and agitated for about 10 hours. The acid is decanted and the sludge washed free of acid. The sludge is dried to about 2% water. The nearly dry powder is screened and pressed into bricks at about 2500 pounds per square inch. The bricks are dried completely and fired at 1600" C. for 2 to 5 hours. The black bricks are then crushed down and screened for the desired abrasive sizes.

The abrasive grain thus produced has .a .composition which varies according to the analysis of the bauxite but which will be within the limits shown in the following table.

Other compounds or, elements Less than 2 2,725,286 Patented Nov. 29, 1955 creased by the fact that the iron oxide includes a large proportion of magnetite, Fe3O4. There is substantially no water of crystallization left in the material; bauxite as mined contains a large proportion of water of crystallization.

A further surprising matter is that this abrasive made as described and having the composition of Table I can be usedto make phenolic resinoid bonded grinding wheels having a materially higher breaking speed than wheels made out of regular fused alumina, the standard abrasive for billet snagging up to now. It is known that the breaking speed for a grinding wheel given in surface feet per minute is substantially independent of the diameter when the central hole is relatively small as is the case with snagging wheels. Hot pressed phenolic resin bonded snagging wheels of hard grades made in a standard man ner and having the standard regular alumina abrasive had a breaking speed of from 20,000 surface feet per minute to 22,000 surface feet per minute. Exactly similar hot pressed phenolic resin bonded snagging wheels of the same hard grades made in the same standard manner and having the sintered calcined bauxite abrasive of Table I had a breaking speed of from 28,000 surface feet per minute to 30,000 surface feet per minute.

For comparison with the calcined sintered bauxite of Table I, I provide analyses of some lots of recently produced regular fused alumina.

TABLE II Percentages, by Weight Lot A Lot B Lot 0 LotD Lot E Lot F Alumina. A1203 93. 60 93. 50 93.50 93. 51 93. 60 Silica. Slog 1.91 1.91 1.87 2.11 1.88 Iron oxide, F920 0.16 0. 14 0. 14 0. 11 0. 13 Titania. T101; 3. 3O 3. 42 3. 42 3. 44 3.31 Lime, CaO--. 0. 13 0. 11 0.09 0. 11 0. 13 Magnesia, MgO 0. 32 0. 34 0.32 0.31 0.35 Zirconla, Z102 0. 47 0.58 0.51 0. 49 0. 42 0. 40

The following tables give analyses of typical bauxites found in various parts of the world excluding, however, the water of crystallization. These tables therefore approximately show the composition of various calcined bauxites all of which can be used in this invention. It will be noted that the percentages do not add up exactly to 100, but they come close and the differences represent minor errors of analysis and calculation. Furthermore evidently many of the samples were not analyzed for CaO of MgO but doubtless all of them contained a small amount thereof.

3 TABLE IV Analysis of five different lots of calcined bauxite Percentages, by Weight. 5

L01: 6 Lot 7 Lot 8 Lot 9 Lot 10 Alumina, A1203 81. 50 85. 10 83. 91. 50 82. 75 Silica. S103 1.60 1.97 .37 2. 44 .75

Iron Oxide, calculated Fez 3. 57 63 7. 35 3. 22 14. 5O 1O Titania, TiOz 11. 90 8. 65 9. 1S 2. 9 1. '30 Lime, CaO 1. 11 trace Magnesia. MgO .41 .52 trace Total 100. 09 99. 87 100. 10 99. 85 99. 99

TABLE v Analysis of five difierent lots of calcined bauxite TABLE VI Analysis of five difierent lots of calcined bauxite Percentages, by Weight Lot 16 Lot 17 Lot 18 Lot 19 Lot 20 Alumina, A1203 72. 65. 20 65.40 68. 3 85. 50 Silica, SiO .88 1.12 3.16 9. 20 4. 26 Iron Oxide, calculated F6203. 23. 21. 61 4. 51 Titania, TlOQ 4 Lime, CaO -z Magnesia, \IgO .43

Total 100. 08 99. 92 98. 99. 89 99. 27

Bauxite, as mined, contains a large proportion of water of crystallization. The various bauxites whose analyses are given in Tables III, IV, V and VI, Lots 1 to 20 inclusive, had originally from about 10% to about 33% of water which, however, was driven off by the calcining process so that these tables give analyses of the calcined bauxite. Bauxite, as mined and also after cal- I cination, contains some iron oxide, as given in the tables. But this is different iron oxide from the iron oxide of the regular fused alumina (Table II). In the bauxite a large percentage (at least one half) of the total iron oxide is magnetite, Fe3O4. In the pigs of regular fused 55 alumina there is present iron oxide FezOz and also some magnetite, Fe3O4, and also some elementary iron; the magnetite F6304 and the iron are, however, removed in the magnetic separating process, so that the final abrasive material contains substantially none thereof. Table 11 reports the analyses of six lots of regular fused alumina abrasive from which the magnetite and the free iron have been eliminated.

Thus the abrasive used in this invention is calcined bauxite which has never been fused, but which has been sintered at a temperature of at least 1550 C., for at least two hours. It can be sintered at higher temperatures and for a longer time provided it is never fused (melted).

The abrasive according to this invention is not corundum under another name (e. g. Upper Hessian bauxite). Corundum is an abrasive which is used for the manufacture of grinding wheels without being sintered. The steps of grinding (ball milling) the bauxite into fines and then pressing the fines into bricks are important steps without which, or the equivalent, the abrasive of this invention cannot be produced. These steps permit the sintering to produce fine grained strong bricks (of substantial size, e. g. 6" x 3" x 2") which are later crushed to produce the abrasive particles.

The manufacture of phenolic resin bonded grindingwheels out of abrasive has been abundantly described and is well known. I prefer to make hot pressed wheels for billet grinding operations; this is done by thoroughly mixingreactive phenol-formaldehyde powdered resin with the abrasive, placing the mixture in a mold, and pressing while heating to resin curing temperature which is usually around 175 C. Steam heated hot presses are used. Such wheels have very low porosity, around 2% or less. They can be described as wheels consisting'of abrasive bonded with thermo-irrcversible phenol formaldehyde resin. Fillers can and preferably will be used; such fillers in the form of fines are simply mixed with the resin and abrasive prior to loading the mold. Typical fillers are cryolite, iron pyrites and potassium fluoborate; the latter two are frequently used in combination. See U. S. Patents Nos. 2,308,981; 2,308,983; and 2,408,319.

A practical test of the quality of a grinding wheel is to square the amount of material removed in pounds and to divide this square by the loss of the wheel by wear measured in cubic inches. A number results which is known as the quality number, and the more economical the wheel (taking into consideration the operators pay, shop overhead and cost of the wheel) the higher the quality number. The comparison is made between wheels of the same size, and of the same ingredients in the same proportions except for one variable being evaluated. Grinding tests on wheels according to the invention operated at high surface speeds (peripheral velocities) and great pressure gave quality numbers as high as 75. The quality number of the best similar wheels made out of regular fused alumina abrasive (Table II) at the highest commercial speed and highest commercial pressure is from 35 to 40. This is when using the hand controlled snagging machines referred to as swing frame grinders. This quality number of the old regular fused alumina wheels could, by increasing the pressure to a certain value, be raised to 50. Raising the pressure above that certain value caused excessive wheel wear which lowered the quality number. It has already been pointed out that the wheels of the invention can be run at higher speeds than would be safe for wheels formerly made. Thus wheels according to the invention will give superior performance in the power operated and controlled, billet grinders previously referred to.

EXAMPLE 1 Two wheels each sixteen inches in diameter by two and one-half inches thick with six inches diameter central holes were molded in a hot press for one hour at about two and one-half tons per square inch and at 160 C. temperature. The wheels were then removed from the press and from the molds and were baked in an oven at temperatures rising from room temperature to C. in four hours then at 140 C. for eighteen hours. These wheels were made from a mixture of the calcined sintered bauxite above described, in a mixture of particles of from No. 12 to No. 16 grit sizes, reactive powdered phenol formaldehyde resin of the brand BR 2417 or equivalent, and fillers, in the proportion of 54 volume percent abrasive, the remainder bond and fillers. The wheels had no pores. Of the 46 volume percent bond and fillers there was 55 volume percent of phenol formaldehyde resin, 21 volume percent iron pyrites, 21 volume percent potassium fiuoborate and 3 volume percent lime, CaO. The iron pyrites and the potassium fluoborate KBF4 are grinding promoting agents, the C210 is a dehydrating agent eliminating the water liberated during curing.

The wheels of Example I were (successively) mounted in the snagging grinder of U. S. application Serial No. 285,423 above referred to and used to grind stainless steel, all grinding being done on steel of the same composition and physical characteristics. The wheels were marked 1 and 2 for identification although they were exactly alike so far as it was possible to make them. In the following table is given the results of the grinding tests, at diflerent wheel speeds as measured in surface feet per minute, where the left hand column identifies the wheel, the next column gives the peripheral velocity in surface feet per minute, the next column gives the pressure between the Wheel and the steel in pounds, the next column gives the wheel wear in total cubic inches lost, the next column gives the steel removed in total pounds, and the right hand column gives the quality, number to the nearest integer derived by squaring the material removed in the above units and dividing this square by the wheel wear in the above units.

TABLE VII Grinding results The figures in Table VII were based upon four fifteen minute grinding tests except for the last (bottom) two. The next to last (next to bottom) figures are based upon one fifteen minute test and the last (bottom) figures are based upon one twelve minute test. The reason for shortening the test at the end is that the wheels had been considerably reduced in diameter and below a certain size grinding is difl'icult.

In general usually more than fifty percent of the iron oxide in the abrasive of the invention is magnetic iron oxide, Fe3O4, but the iron oxide is calculated as FezOs as is conventional. Fused alumina with the iron and most of the iron oxide removed has a hardness about that of sapphire, which hardness is stated as 9 on Mohs scale by definition according to Mohs.

It will thus be seen that there has been provided by this invention an abrasive and a grinding wheel in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth is to be interpreted and not in a limiting sense.

I claim:

1. A grinding wheel consisting of abrasive consisting of sintered material derived from calcined bauxite reduced to a fine powder, pressed into masses and sintered at 1550 C. for at least two hours, said abrasive having a composition of Percentage range, by weight Alumina, A1203 to 97 Silica, SiOz .3 to 16 Iron oxide calculated as Fe2O3 4 to 35 Titania, TiOz up to 12 Lime, CaO up to 1.5 Magnesia, MgO up to 1 Other compounds or elements less than 2 Water of crystallization substantially none.

said abrasive having a hardness close to 9 on Mohs,

Alumina, A1203 65 to 97 Silica, SiOz .3 to 16 Iron oxide calculated as Fe2Oa-.. 4 to 35 Titania, TiO: up to 12 Lime, CaO up to 1.5 Magnesia, MgO up to 1 Other compounds or elements less than 2 Water of crystallization substantially none said abrasive having a hardness close to 9 on Mohs scale and a fine crystal structure and having a strength greater than that of fused alumina, the major portion of the iron oxide in the abrasive being magnetic iron oxide, F6304, said abrasive being bonded with thermo-irreversible phenol formaldehyde resin.

4. A grinding wheel according to claim 3 having a porosity of less than 2%.

References Cited in the file of this patent UNITED STATES PATENTS 2,256,528 Rowe et al Sept. 23, 1941 2,278,442 Heany Apr. 7, 1942 2,311,228 Heany Feb. 16, 1943 2,363,146 Roble Nov. 21, 1944 2,441,534 Norton May 11, 1948 

1. A GRINDING WHEEL CONSISTING OF ABRASIVE CONSISTING OF SINTERED MATERIAL DERIVED FROM CALCINED BAUXITE REDUCED TO A FINE POWDER, PRESSED INTO MASSES AND SINTERED AT 1550* C. FOR AT LEAST TWO HOURS, SAID ABRASIVE HAVING A COMPOSITION OF 